Traveling wave electron tube



March 25, 1958 w. J. DoDDs ET Ax. 2,328,440

TRAVEL-ING WAVE ELECTRON TUBE Filed June 22, 1950 3 Sheets-Sheet 1 l! Ijl/ ATTORNEY March 25, 1958 w. J. DoDDs Erm. 2,828,440

TRAVELING WAVE ELECTRON'TUBE Filed June 22, 195o s Smets-sheet g wfufafnrmw g Mou M pfff/ ATTORNEY March 25, 1958 w, J, DoDDs ErAL 2,828,440

TRAVELING WAVE ELECTRON TUBE v /jlllllllllli f 49 6/ l i! il /g 4 Mmmm @W @w Mw /f/ if United States Paten-tl O TRAVELING WAVE ELECTRN TUBE Wellesley I. Dodds, Granbury, and Rolf W. Peter, Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application June 22, 1950, Serial No. 169,674

v Claims.r (Cl. S15-3.6)

This'invention relates to improvements in,A electron tubes, and particularly to beam, tubes. ot" the traveling wave type.

In alconventional traveling wave-tubeanl elongated v sectionA of transmission line ismounted within a vacuum envelope: with suitable'input and. output structures` sealed through the envelope and coupledfto the opposite ends of the section. The transmission line section is designed asY a delay line along; which electromagnetic'waves are transmitted at a fraction of the velocity of light. Although. other forms have been suggested, the form of delay line usually used intraveling wave tubes is a. con.- ductive helix, such. asV a helical metal coil. An electron beam is projected Vby suitable means along, and preferably coaxiolly of, thev helix at a beam velocity substantially equal to the: axial wave velocity alongthe helix. In 'operation of the tube as an. amplifier, a signal vwave travelingalong the helix creates electromagnetic fields therealongwhich interact with the electrons in. the beam to produce electron velocity modulation. and consequent electron bunching. As the wave and beam travel synchronously along the helix the phenomenon reverses-and the. hunchedbeam induces elds and currents along; the heli-x. Theamplitude of the wave increases exponentially along thehelix, because the electron beam; gives` upg-.more energyA to the helix than it abstracts therefrom, thus. producing an amplified. signal at the output end'ofjthetube.

The axial velocity of a wave'alonga simple helical coil, designed for very high'frequencies and having practical diameter and pitch is: determined largely by* the diameter and. pitch of the helix, and. the velocity of light, ,ann is substantially independent of the: frequency-ofthe wave: transmitted. T herefore thev conventional trave-ling wave tubedescribed is a very broad; band'am'plier, the useful bandwidth ot the tube being limited primarily onlyVA by the bandwidth of the input and outputstructures.

ln=` the: use of; traveling Wave tubesV of. the helix type it oftenV happens that the amplication. bandwidth of the tube is manyl times. greater than that necessaryI or even desirable for the. adequatetransmission of. intelligence. This characteristic: can. causev serious-.problems to arise wi-t-hthe high-frequency circuits associated with thettube. If the pass bandwidths. of the: circuit elements. coupled totheinputand. output ends of the helix are not'at least comparableA to, the tube bandwidth, noise fluctuations in the. electron beam. will be reilected at theseJ elements and cause regenerative oscillations.. Also, when it is desired to. arbitrarily limit. the bandwidth of a given helix tube by operations upon the input and. output circuits, in order toreiect unwanted signals, such reflections make it: impossible `to do so without` regeneration.

The principal object of the present inventionl is,wthere fore, to provide improved means'associatedl with the helix for limiting'. the. amplificationY bandwidth of Vai helix. type traveling, wave. tube.

A11-important. feature ofthe invention is the-provision oi bandwidthlimiting means uniformly distributed along the` length ofthe helix..

Mice

Another featurev of the invention is the provision-of such bandwidth limitingmeans in the form of periodically distributed, substantially non-resistive inductance elements, either in contact with, or forming an integral part of, or closely coupledto vtheactive helix of the tube.'

In general, ther bandwidth limiting means employed may beany structure vorstructures associated withthe helix, which will produce periodically-recurring inductance changes'along thehelix. The effect of .these changes is tlo.produce-ineriodic variations in characteristic impedance along the helixrand'give the helix dispersive4 qualities.

Thief-wave velocity' along such a helix; is no longer independentv of frequency; Instead, the tube isin effect pre- -tunedduringzmnufacturewto a particular. frequency, or

hand. of frequencies; 1 ata s given beam- .volt-age..

These.` and. otherv objects,;. features. and advantages.. of the invention will be apparent tothose skilled in=y the art fromrthe; following' detailedl description ofv the invention taken in: connection-with the annexed drawing, in which.:

Fig, l is agraphA showingr the relation between tube gainand beanrgvelocityfor a; helix traveling wavev tube;

Fig.; 2 is:,a schematic view of 'a transmission linefin the .form of ia. laddennetworkg..

Eig.A 3 is-xa'. longitudinalsectional view of a traveling wave: tubeembodyingrone: form. of the invention;

Eig.14 is a; transverse sectional view taken on line 4-L-4 of Figa; 3;..

li'g;v 5 isza detailfviewr showing; a modification of the tube of .Fig..,3;,.

Fig.. 6-:.is a; schematicview `of another embodimentof theinveution; g

Figs. 7 82 and.9fare`.longitudhralsectional; views of tlrree; other rnodilications;v

Figs.: L01 and.` 1:1 are .elevational viewsrrof two forms/of helices thatmaybe used-in.` placeof. the .helixotl-ieg.. 8g

for example;

Eig.. 1.2; is. a longitudinal sectionalfdetail-view showing another;modicationaandz Fig.. 16: is.y a transverse= sectional'view taken oneline 13-13 of Fig. 12.

Referring,V to the drawing, Fig.; li showsthe'- general relation between. tube gain and Jbeamcfvelocity' in aconventional helix-type vtraveling wave amplifier tube; The curve shows that the tube gain is: .greatest `when ithefbeam velocity is. adjustedto av value slightlyl greater thanl the axial wave velocityy vi,L for the particular helilx used, and that; the gain falls: offfif there is: any'substantialdisparit-y between: theA beam velocity audi the` helixV wave velocit-5L. However, signals-.frequencies Vover a;v very* wide band'are amplilieds substantiallyY equally.. curve showing the relation betweeni tube gain and: the reciprocal of'wave velocity for a given Iconstant beam velocity vowould havev the same' generalshape asfth'e curve ofFig. l'. It can be seen that if a tube were designed: so that, for a given beamvelocity, a'- desired signal frequency-would be ampl'ilied' at theV peak of again curve, such vas shown in Fig. l', and othensignal frequencies would haverlower or higher-#wavev velocities along the'- helix', depending upon whether their frequencies were higher or 'lower than the desi-red? signal?, these other' undesiredsignalsf would experience less gain or even arnet attenuation, and, conf se'quently-,the operating bandwidth of the'ampliiier would be limited'.

Limited bandwidth operation has been realized in the pastbyf use of="heli`ce s which' were dispersive as a` result of having their diameters smallcompared to av wavelength and relatively open pitches. However, such a' tube is consideredimpractical" because such design for'helix dispersion at microwaves leads-*toundesirably high bea'm Voltage; tubes or to-loW-voltagez tubes with excessively small helix. diameters and extremely fine helixl wire.

In vaccordance withA therpresent; invention;` theahelix` of- 'on the tube gain, as shown Vin Fig. l, isV equivalent to shifting the point of operation, for a particular beam velocity,

away from the region of maximum gain in the case of W signals having helix wave velocities diiferent from v0.`

In carrying out the invention, the helix is designed to make it similar in structure and function to a ladder networ used in transmission lines in general. A suitable ladder network is schematically shown in Fig. 2 as a transmission line made up of successive portions of different characteristic impedance. The portion of the line between a-a' and b-b has a length L1 and characteristic impedance Z1; the portion between b--b and c--c' has a length L2 and characteristic impedance Z2; the portion between c-c and d-d has the same length and characteristic impedance as the first mentioned portion, that is, L1 and Z1; etc.

Figs. 3, 4 and 5 show two embodiments of the invention in which a lter structure surrounding the helix along its length is used to give the helix a dispersive character. In Figs. 3 and 4, a conventional elongated metal helix or helical coil 1 of desired diameter and pitch is supported in spaced coaxial relation within a metal filter structure 3 by means of longitudinally-extending dielectric rods 5 disposed around the helix, as shown. The lter structure 3 is made up of alternately-arranged, apertured metal discs 7 spaced apart and connected by intermediate metal rings 9. The discs 7 are provided with circular notches around the inner wall to receive the rods 5 which support the helixl. The inner surfaces of the discs 7 and rings 9 provide conducting surfaces located at different distances from the helix 1 and distributed uniformly along substantially the entire length thereof. The thickness of each of the discs and rings is the same, and is preferably a quarter wavelength at the desired operating frequency. The discs and rings are sealed together to form part of the evacuated envelope of the tube. Suitable direct-current operating potentials are applied to the various electrodes, by a battery 10, for example.

Any suitable means may be used to couple an input signal wave to the helix 1. A waveguide 11 is shown coupled to an axial extension 1 of the helix and connected to the metal filter 3. The waveguide is sealed at 11'. A cup-shaped glass envelope portion 12 sealed to the waveguide 11 contains a gun structure comprising a cathode 13 and beam-forming electrode structure 15 for projecting an electron beam along and in energy-coupling relation with the helix 1. An output waveguide 17 is' coupled to an axial extension 1" of the other end of the helix 1 and connected to the filter structure 3. The waveguide 17 is sealed at 17. A collector 19 is mounted within `a cup-shaped glass envelope portion 21, which is `sealed to the waveguide 17.

Each of the sections of the ilter structure 3, that is, each pair of adjacent discs 7 and the connecting ring 9, comprises an inductance element capacitively coupled to the helix 1 at two axially-spaced points therealong. The etiect of the filter 3 on the operation of the tube is to produce periodically-recurring inductance changes which produce changes in characteristic impedance along'the helix, the characteristic impedance in the region ofeach disc 7 being diierent from that in the region of each ring 9. This gives the helix a dispersive character, and effectively pre-tunes the tube to a predetermined frequency, or narrow band of frequencies. The filter also serves as an electrostatic shield for the helix 1.

, 'The means shown in Fig. 3 for obtaining dispersion and limited bandwidth in the helix traveling wave tube becomes diicult when it is desired that the rejection bands of the lter be made very wide, and the pass bands be made .very narrow. This condition arises fronrthe fact that in order to have narrow pass bands, the effective ratio between the two different segments of the filter must be made very high, and this requires that the filter periodically come extremely close to the helix without making actual physical contact.

Fig. 5 shows a modification o-f the tube of Pigs. 3 and 4 wherein the apertured discs 7a are of dielectric material and the helix is supported by the discs. The discs 7a are spaced apart by spacer rings 9a which may be of metal. This construction is equivalent electrically to that of Figs. 3 and 4, and eliminates difliculties of insulating the helix from a closely spaced metal filter. In this embodiment, a separate glass envelope 23 is preferably provided around the composite glassandmetal lter structure. The input waveguide 25 surrounds the envelope 23, as shown.

Each of the filter structures of Figs. 3-,5 may be considered as providing a plurality of distributed inductance elements uniformly distributed along the length of and capacitively coupled to the helix 1. A similar effect can be produced by lumped inductance elements periodically distributed along and connected directly to the helix. Fig. 6 illustrates schematically an arrangement of reactance elements 27 in groups and connected between the helix 1 and a shield 29 surrounding the helix. The spacing between adjacent groups is uniform, and preferably,

Ythe spacing of the individual reactance elements 27 in each group is also uniform. The effective axial length of each of these reactance elements is small compared to a wavelength along the helix. The reactance elements 27 of Fig. 6 produce inductance changes along the helix 1 because of their connections to the helix which consti- `tute inductance elements. The elements 27 produce regions between the groups which have one characteristic impedance and regions within the groups which have a different characteristic impedance, which gives the eect of a ladder network.

It is not necessary that the localized inductance elements be arranged in groups as shown in Fig. 6. Single inductance elements will produce small regions of different characteristic impedance. Fig. 7 shows a tube structure embodying one form of single localized inductance element which may be used. The tube comprises an elongated metal shield 31, which also serves as a part of the tube envelope. An elongated metal helix 1 of constant diameter and pitch is coaxially mounted in spaced relation within the shield 31. The vacuum envelope of the tube is completed by cup-shaped glass members 12 and 21 having their open ends sealed to the shield 31. A'waveguide 37 is coupled to an axially-extending end 1' of the helix 1 and connected to the shield 31. The output end of the helix 1 is coupled to an output waveguide 39 coupled to an axially-extending end 1" of the helix and connected to the shield 31. The electron beam is collected by a collector electrode 19 mounted adjacent to the output end of the helix. The waveguides 37 and 39 are sealed at41 and 43, respectively.

In the embodiment of Fig. 7, a plurality of coaxial line stubs 45 with inner conductors 47 are uniformly distributed at equally-spaced points alongthe helix 1, each stub 4S being connected between the helix 1 and the shield 31, as shown. Thedistance along the helix between the inner conductors47 of adjacent stubs would usuallyrbe a half wavelength, but in particular instances the distance might be some other value, the important thing being the equal spacing therebetween. The electrical length of each stub 45 is not critical, in makngthe helix dispersive. However, the length may be a quarter wavelength, for parallel resonance, at the desired signal frequency to by-pass other unwanted signal frequencies to the shield. On the other hand, the stubs 45 may have a length of a half Wavelength, for series resonance, at some unwanted signal frequency to by-pass this particular frequency. The stubs 45 may have a length to make them inductive at the desired signal frequency. Lumped inductance circuit elements may be substitutedforthe distributed-inductance stubs 45, coupled between the helix 1' and the shield 31. The inner conductors-I7 ofthe stubs or other inductance elements produce periodicallyrecurring inductance changes along the` hel-ixI at their points of connection to the helix. If a` fairly stiff helix is used, it may be supported within the shield 31 solely by the inner conductors 47 of the stubs 45. If'necessary, separate insulating supports such as are shown in Fig. 8 may be provided.

It is a well-known that the helix of a traveling wave tube functions primarily as a single-conductorl transmission line. Hence, the usual shield about the helix is not essential to complete the high frequency circuit of the tube, and may even be omitted in cases where it is not purposely coupled to the helix as in Figs. 3, 5, 6 and 7. In accordance with the present invention, any discontinuity in the structure of the helix will produce a change or discontinuity in the characteristic impedance of the helix line, which will cause retiections of waves traveling therealong, at that point. The localized reactance elements 27 in Fig. 6 and 45 in Fig. 7 constitute such discontinuities in the structure of the helix 1, at their points of connection to the helix. When these discontinuities are periodically distributed at equal intervals along the helix they cause the helix to be dispersive, and hence, limit the amplification bandwidth of the tube to a narrow band of frequencies. It will be understood that the discontinuities in the structure of the helix itself, including conductors connected directly thereto, as shown in Figs. 6-9, ll and 12 constitute reactances that are primarily inductive, rather than capacitive.

Figures 8 through 13 show some other forms of localized inductance elements which may be provided, in place of the stubs 45 of Fig. 7, to produce the desired inductance changes along the helix. In Fig. 8, groups of conductive tabs 49 are attached to the helix 1 at equal intervals therealong. Each of the tabs 49 constitutes an inductance element. The envelope 51 is entirely of glass, and the shield 53 and input and output structures 55 and 57, respectively, are disposed outside the envelope 51. There is no appreciable `coupling between the shield 53 and the helix 1 in Fig. 8, and hence the shield may be omitted, especially where the helix pitch is relatively small. The helix 1 may be supported coaxially within the glass envelope 51 by means of a plurality of longitudinally-extending dielectric rods 59 distributed around the helix, as shown in Fig. 8.

In Fig 9, single tabs 61, similar to the individual tabs 49 of Fig. 8, are arranged on opposite sides of and attached to the helix, with longitudinally-extending legs 63 engaging the inner wall of the envelope 51 to support the helix 1 coaxially therein. The legs 63 also cooperate with the shield 53 to provide some capacitive coupling between the tabs 61 and the shield 53, when the shield is provided. In this respect the arrangement of Fig. 9 is analogous to that of Fig. 7.

The structure of the helix itself may be modified to give it dispersive properties. This may be done, as shown in Fig. 10, for example, by constructing a helix 65 with periodically distributed changes in pitch. The helix may be made with alternate portions 67 of equal length and one predetermined pitch connected by intermediate alternate portions 69 of equal length but another predetermined pitch. The changes in pitch produce periodically distributed changes in inductance and characteristic impedance along the helix causing the helix to be dispersive. A similar effect can be produced by constructing the helix with periodic changes in wire diameter, instead of pitch. Fig. 11 illustrates a helix 1 provided with conductors 71 connected between and shorting adjacent turns 73 at equal intervals therealong. The conductors 71 act as inductance elements attached to the helix by producing changes in thewave .path along the helix.. It will be understood 'that more than two adjacent turns I3of the'helix'may' be Ain that the shortingf'conductors` 71v produce` effective changes in pitchv along the helix.

Fig. l2 shows a helix 75', of relatively larger diameter than those previously. disclosed, snugly litted within a glass envelope 51. Coaxially mounted Within the helix 75 are a plurality of rings 77 surrounding the. beam path 79 andsupportedby. an connected tothe helix by rods 8.1 distributed at equal distances. along thehelix. Preferably, but not necessarily, the. rings. 77 are. attached to the. helix at every turn, in View of thelargehelixdiamcter. The rings 75 and Vrods 8.1 not.only-.provide. inductance elements uniformly spaced along the helix but also greatly enhance the coupling between the beam and the helix, particularly for helices whose diameters approach the order of magnitude of the wavelength therealong. The structure illustrated in Figs. 12 and 13 is the sole invention of Wellesley I. Dodds, and is claimed in his cepending application Serial No. 305,797, led August 22, 1952, now Patent No. 2,802,135, issued August 6, 1957.

The invention, therefore, provides means, in the form of distributed or lumped inductance elements periodically distributed along the length of the helix of a traveling wave tube, for limiting the bandwidth of the tube when used as an ampliiier, by producing periodically-recurring variations in characteristic impedance along the helix.

In the claims, the term directly connected to is intended to be limited to a low-impedance direct-current connection, as distinguished from a capacitive or an inductive connection. Each of Figs. 6 through 10, 11 and 12 shows conductors which are directly connected to the helix at spaced points therealong.

It will be apparent that the invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structures used and the purpose for which they are employed without departing from the scope of the invention as set forth in the following claims.

What is claimed is:

1. An electron tube of the traveling wave amplifier type comprising an elongated continuous conductive helix, means adjacent to one end of said helix for projecting a beam of electrons along and in energy coupling relation with said helix, and means for limiting the amplification bandwidth of the tube comprising discontinuous means distributed along substantially the entire length of said helix for producing substantial inductance changes along the helix.

2. An electron tube as in claim 1, wherein said helix has constant diameter and pitch, and said last-named means comprise a series of inductance elements coupled to the helix at spaced points periodically distributed therealong.

3. An electron tube as in claim 2, wherein sai-d spaced points are equally spaced .along the helix.

4. An electron tube as in claim 2, wherein said inductance elements are directly connected to said helix.

5. An electron tube as in claim 4, wherein said inductance elements are arranged in similar groups uniformly spaced along said helix.

6. An electron tube as in claim 2, wherein said inductance elements are provided by shielding means surrounding said helix and including conductive portions uniformly spaced along substantially the entire length of the helix and located at a predetermined distance therefrom .and intermediate conductive portions spaced further from said helix than said first named portions.

7. An electron tube of the traveling wave amplier type comprising an elongated continuous conductive helix having constant diameter and pitch, means adjacent to one end of said helix for projecting a beam of electrons nleans for limiting the amplication bandwidth of the tube comprising a series of vconductive tabs attached directly to said helix at spaced points periodically distributed along substantially the entire length thereof, `for producing substantial inductance changes along the heliX at said spaced points. Y A

8. An electron tube as in claim 7, further including a dielectric envelope surrounding said helix, said tabs having portions engaging the inner surface of said envelope for supporting said helix within the envelope.

9. An electron tube as in claim 7, wherein each of said tabs is connected to at least two adjacent turns of said helix for producing periodically distributed changes in the effective pitch of the helix.

r10. An electron tube as in claim l, wherein said inductance changes are produced by periodically distributed changes in the elective pitch of said helix.

References Cited in the le of this patent UNITED STATES PATENTS Hael Dec. 15, 1936 2,122,538 Potter July 5, 1938 2,511,407 Kleen et al. June 13, 1950 2,531,972 Doehler et al. Nov. 28, 1950 2,578,434 Lndenblad Dec. 11, 1951 2,672,572 Tiley Mar. 16, 1954 

